This invention relates generally to tar sands extraction and, more particularly, to a process for the removal of mineral fines from tar sands extracts.
In view of the recent instability of the price of, and consumer country access to, crude oil, researchers have renewed their efforts to find alternate sources of energy and hydrocarbons. One of the possible sources for at least a portion of our energy needs is tar sands, also commonly referred to as oil sands or bitumen sands. Tar sands are generally characterized as comprising a consolidated or partially consolidated porous mineral structure, e.g., sandstone, which contains a high proportion of bitumen, i.e., a three component system of oils, resins and asphaltenes, with each of these components typically having successively higher solubility parameters. Thus, the bitumen consists of a mixture of a variety of hydrocarbons and, if properly separated from the sand or mineral component, can be used as a feedstock for the production of synthetic fuels and/or petrochemicals. For example, the tar sand deposits of the intermountain region of the western United States have an estimated reserve of more than twenty-eight billion barrels of oil in place. Although this resource is only a small fraction of the total United States oil requirement, it could be an important source of hydrocarbons on a regional basis.
The nature of tar sands varies greatly depending upon their geographical source insofar as certain tar sands deposits are more easily processed than others. For example, Athabasca tar sands from Alberta, Canada, have an average bitumen content of about 12-13 weight percent and a relatively high moisture content of about 3-5 weight percent. It is believed that these tar sands consist of aggregates of sand, wherein each grain of sand is surrounded by a film of connate water, which separates the bitumen from the sand grains. This structure permits a relatively easy separation of the bitumen from the mineral component of the tar sands, even when such tar sands are processed on a large scale. In fact, commercial hot water extraction processes for recovering bitumen from Athabasca tar sands presently exist. A good review of the Alberta tar sands projects is presented in an article entitled "Tar Sands: A New Fuels Industry Takes Shape," Science, Vol. 199, page 756 (February 1978).
On the other hand, most deposits of tar sands found in the intermountain region of the U.S. have an average of less than about 10 weight percent bitumen and negligible amounts of connate water, hence the bitumen is in direct contact with the grains of sand. This situation makes recovery much more difficult. Examples of such tar sands include the Sunnyside and Asphalt Ridge deposits found in Utah.
A further type of tar sand found mostly in California is the diatomaceous earth type, which contains up to about 25 weight percent bitumen. In this type of deposit the bitumen is contained within the very fine pores of the matrix and consequently is generally relatively difficult to extract. Such deposits typically also yield a large amount of mineral fines upon extraction.
In addition to a variety of aqueous extraction processes, other extraction processes have been disclosed which, among other features, use one or more of a variety of solvents. For example, U.S. Pat. No. 3,941,679 discloses the use of trichloromethane as an extraction solvent. U.S. Pat. No. 4,036,732 discloses the use of paraffinic hydrocarbons having from 5 to 9 carbon atoms. U.S. Pat. No. 4,046,663 teaches the use of a naphtha/methanol solvent system.
In selecting a solvent system for tar sands extraction, a number of factors are generally considered in evaluating performance. An obvious factor in evaluating the performance of a solvent system in tar sands extraction is the effectiveness of the solvent in separating the bitumen from the sands. This is often counterbalanced by a second consideration, however, which is the asphaltene content of the recovered bitumen. Asphaltenes are complex high molecular weight hydrocarbons which may be undesirable in particular subsequent refining processes. In this regard, an article entitled, "A Solubility of Asphaltenes in Hydrocarbon Solvent," by D. L. Mitchell and J. G. Speight, Fuel, Vol. 52, pp. 149-152 (1973), extensively explores the solubility of asphaltenes for over 50 different solvents and blends.
A third factor of importance is the fines or mineral particle content of the extracted bitumen. Mineral fines initially present in the tar sands as well as mineral fines formed, such as during grinding operations of a tar sands sample in preparation for extraction recovery of bitumen from the sample, pose difficult downstream processing, e.g., pumping, mixing, separation, etc., problems. The rate at which such fines settle is to a large extent dependent upon the solvents used and has been thought to be primarily determined by the density and viscosity of the solvent.